This invention relates to a method for preparing a copolyestercarbonate comprising at least one carbonate block and at least one polyester block with chain members derived from at least one dihydroxy-substituted aromatic hydrocarbon moiety and at least one aromatic dicarboxylic acid moiety (sometimes referred to hereinafter as arylate chain members). In a particular embodiment the invention relates to a method for preparing a copolyestercarbonate comprising at least one carbonate block and at least one polyester block with chain members derived from at least one 1,3-dihydroxybenzene moiety and at least one aromatic dicarboxylic acid moiety (sometimes referred to hereinafter as resorcinol arylate chain members).
Various resinous articles comprising polymers have a problem of long term color instability. In many cases this instability is seen as yellowing of the polymer, detracting from the article""s attractiveness and also transparency when the polymer was initially transparent. Loss of gloss can also be an undesirable long term phenomenon in a resinous article.
Yellowing of polymers is often caused by the action of ultraviolet radiation, and such yellowing is frequently designated xe2x80x9cphotoyellowingxe2x80x9d. Numerous means for suppressing photoyellowing have been employed and proposed. Many of these involve incorporation in the polymer of ultraviolet absorbing compounds (UVA""s). For the most part, UVA""s are low molecular weight compounds, which must be employed at relatively low levels, typically up to 1% by weight, to avoid degradation of the physical properties of the polymer such as impact strength and high temperature properties as reflected in heat distortion temperature. Another problem of concern with polymers such as aromatic polycarbonates and addition polymers of alkenylaromatic compounds such as styrene is susceptibility to attack by organic liquids.
One way of protecting a resinous article against photoyellowing and loss of gloss is to apply a coating of a weatherable second polymer, the term xe2x80x9cweatherablexe2x80x9d as used herein signifying resistance to such phenomena. Coatings made from polyesters containing resorcinol arylate chain members often possess good weatherability properties. The arylate moieties typically contain isophthalate, terephthalate, and especially mixtures of iso- and terephthalate units.
The good weatherability properties of polyesters containing resorcinol arylate chain members are believed to arise in large part from the screening effect said chain members may provide to ultraviolet (UV) light. On exposure to UV light polymers comprising resorcinol arylate chain members may undergo photochemical Fries rearrangement converting at least a portion of said chain members from polyester-type chain members to o-hydroxybenzophenone-type chain members. The o-hydroxybenzophenone-type chain members act to screen further UV light and protect UV-sensitive components in a resorcinol arylate-containing composition. The good weatherability properties of polymers comprising resorcinol arylate chain members make them especially useful in blends and in multilayer articles in which said polymers may act as a protecting layer for more sensitive substrate components.
Copolyestercarbonates often possess property advantages compared to their parent polyester or polycarbonate comprising chain members similar to the corresponding blocks in the copolymer. For example, copolyestercarbonates derived from combination of resorcinol with mixtures of isophthalate and terephthalate chain members may have good weatherability properties and may provide protection against photoyellowing when coated over a resinous substrate. Methods for preparation of copolyestercarbonates are disclosed in, for example, U.S. Pat. Nos. 3,030,331; 3,169,121; 3,207,814; 4,194,038; 4,156,069; 4,238,596; 4,238,597; 4,286,083; 4,487,896; 4,506,065; 5,321,114; and 5,807,965. Block copolyestercarbonates comprising resorcinol arylate chain members in combination with carbonate chain members are disclosed in published PCT Application WO 00/26275. These copolymers have excellent weatherability and are compatible with polycarbonates in blends. In one embodiment these block copolyestercarbonates are prepared by a method which comprises the steps of: (A) preparing a hydroxy-terminated polyester intermediate having a degree of polymerization of at least 4 by the reaction of at least one 1,3-dihydroxybenzene moiety with at least one aromatic dicarboxylic acid dichloride; and (B) conducting a reaction of said polyester intermediate with a carbonate precursor. The polyester intermediate is synthesized by an interfacial method employing in one embodiment unsubstituted resorcinol, isophthaloyl dichloride, terephthaloyl dichloride, water and dichloromethane, which is often the organic solvent of choice. The hydroxy-terminated polyester intermediate comprises phenolic hydroxy end-groups suitable for growing polycarbonate chains in a subsequent reaction step with a carbonate precursor, and optional second dihydroxy-containing compound.
Brunelle et al. in U.S. Pat. No. 6,265,522 teach that the phosgenation step of an interfacial copolyestercarbonate synthesis process can be conducted at an initial pH target of 7.5-8.5, followed by increasing the pH target slowly to 10-10.5. Typically, a total of 15% excess phosgene is added over the stoichiometric amount. This approach requires careful monitoring of the reaction pH. However, in interfacial synthesis, the reading obtained on a pH electrode often deviates (toward pH 7) by one or more pH units from the pH read by indicator paper, which is generally a more accurate indication of the actual pH. Erroneous or unreliable pH electrode readings make it impractical to conduct this type of caustic addition on a commercial scale.
Silva et al. in U.S. Pat. No. 5,973,103 teach a method for preparation of polycarbonate homopolymer in which aqueous caustic is added at a rate that is in a fixed ratio to the phosgene rate. The ratio of caustic to phosgene can be up to 1.8 mole/mole. One objective of this method was to make chloroformate-terminated oligomers of a specific molecular weight in the first stage of a two-stage polymerization process. This was accomplished by permitting only partial conversion of the phosgene through limited caustic addition. The current invention is outside the scope of this teaching.
It remains of interest, therefore, to develop a method for preparing copolyestercarbonates, said method to proceed under controlled conditions of base addition with minimal dependence on pH electrode accuracy. There is also a need to develop a method for preparing copolyestercarbonates while minimizing phosgene usage.
In one of its aspects the present invention provides a method for preparing a block copolyestercarbonate comprising chain members derived from at least one dihydroxy-substituted aromatic hydrocarbon moiety and at least one aromatic dicarboxylic acid moiety, said polymer being substantially free of anhydride linkages linking at least two mers of the polyester chain segments, comprising the steps of:
(a) preparing a hydroxy-terminated polyester intermediate comprising structural units derived from at least one dihydroxy-substituted aromatic hydrocarbon moiety and at least one aromatic dicarboxylic acid moiety; and
(b) conducting a reaction of the polyester intermediate with phosgene in a reaction mixture comprising water, a substantially water-immiscible organic solvent, and a base, wherein base and phosgene are added simultaneously to the reaction mixture at a substantially constant molar ratio of base to phosgene for a time period of at least about 60% of the total amount of phosgene added.